Hi Datwood, and welcome to the avast forums.
1) Currently installed the 'Demo' but when I try and install the Exchange Plug-in Avast! tells me it wont install on the Demo version. Thought it was a fully enabled Demo?
The demo is indeed fully functional. What message are you getting, exactly (a screenshot would be ideal) and when, exactly (how do you get to it)?
2)Re: the Exchange plugin and this post here http://forum.avast.com/index.php?topic=16031.0 Does the exchange plugin not scan the Exchange mailboxes on the server? thats a client side plugin designed to scan e-mail coming from the exchange server to my Exchange-enabled Outlook client? That doesn't make sense to me....
The workstation versions of avast (that is, avast Home Edition, avast Professional Edition and avast NetClient Edition) contain an "Outlook/Exchange" module, which is designed to protect Outlook traffic (i.e. on workstation, not server, level). In other words, this works as a plugin to Outlook.
The Server Editions, on the other hand, contain two additional modules, called "MS SMTP 2000/2003" and "Exchange 2000/2003" that work together to protect Exchange traffic on the server level. That is, these are installed on the Exchange/SBS server itself (and I assume this is what you're trying to do).
3) Also tells me I can't update program files or Virus definitions on the demo version.
Again, the demo should be fully functional and enable normal updating. The updater should show you a log, what does it say? License expired?
4) Purchasing. What exactly does an additional License get me? Desktop installation? Scanning of an additional mailbox per license? Additional Server installations?
Well, of course. It depends on what you buy

. Either you buy only protection for the Exchange server (which is priced per mailbox), or for the whole SBS (this is called "avast! SBS Edition") or you can buy the whole SBS suite which will give you protection for the server and all the workstations.
Thanks
Vlk